Modulation control system



Dec. 19,'1939. o. E. KEALL y2,133,717

MODULATION CONTRL SYSTEM l Filed oct. 2s, 1957 klNvr-:NTQR 0. E. KEALL ATTORNEY Patented Dec. 19, 1939 unirse STATES 2,183,717 MODULATION COTROL SYSTEM Oswold Edward Keall, Essex, England, assigner to Radio Corporation of America, a corporation of Delaware Application October 26, 1937, Serial No. 171,053 In Great Britain November, 1936 1 Claiin.

This invention relates to modulated. carrier .wave transmitting apparatus and more particularly to automatic modulation monitoring systems for use in radio and like transmitters. By the term automatic modulation monitoring systems as employed in this specification is meant systems wherein modulating signals. are automatically controlled in value so as to prevent overloading or under-loading (or both) of a transmitter,

Manually operated modulation monitoring sysltems are Well known in connection with radio systems, but such systems oiier the serious defect that in practice it is not possible to reduce an excessive degree of modulation until after overmodulation has in fact occurred a time lag of two or three seconds being quite usual. Further, in the case of radio transmitters modulated with music signals, there is the-difhculty that certain forms of over-modulation, for example overmodulationljdue to percussion eiects in an orchestra, which last but a fraction of a second, and which may result in the 4tripping of a circuit breaker and thus interruption of the service, are almost impossible to control `manually unless pre- Vious warning is given. y l

The object of the present invention is to overof said wave and means including alight sensitive I ydevice and associated light control device for automatically controlling the modulation input to the transmitter to be modulated in dependence upon the length of said sweep, The light control device may, for example, be a mask or-shutter or it may be a so-called density wedge.

The invention is illustrated in the 'accompanying drawing which shows schematically one embodiment thereof as applied to an ordinary radio transmitter.

Referring to the drawing the automatic modulation monitoring system therein illustrated comprises a high frequency amplifier I which is fed with and amplifies modulated carrier wave derived from a suitable point in the transmitter. For example, asshown, lthe high frequencyamplifer I may be fed from a 'receiving aerial 2 which is radiation coupled with the transmitting aerial t of the transmitter 4 to be monitored. The output from the high 'frequency amplifier l is fed to a pair of delector plates 5, 6 in a cathoder ray oscillograph tube 'I having a fluorescent screen on its end wall 8. It will be appreciated that with this arrangement the high frequency output from the amplifier I will deflect the cathode ray in the cathode ray tube l and produce upon the screen at 8 a straight line whose length will depend on the degree of modulation in the carrier wave. Positioned to be activatedby light from the screen ofthe cathode ray tube is a photo-electric cell 9 and between the said cell and the said .uorescent screen at 8 is a suitably dimensioned and shaped mask or shutter il! which is constructed in accordance with the degree and naturev of the monitoring control desired. In place of using a mask4 or shutter at I0 a socalled density wedge or combination of density Wedges may be employed. One Well known form of density wedge" consists of a piece of plane glass coated with a light sensitive emulsion which has been exposed to light and suitably treated thereafter so that the density or transparency of the wedge varies over thelength thereof in a desired predetermined manner. The output from the photo-electric cell 9 is ampliiied by a direct current photo-cell amplier II the output from which is utilized to control the modulation input to the transmitter; for example,

the said output may be utilized as gain controlling grid bias upon one or more tubes in an audio .4 frequency amplier I2 interposed between the sound pick-up microphone 'I3 or other modulation signal source of the transmitter and the rest ci the transmitter at 4. 'I'he apparatus shown in the ligure between the broken lines I4, I5 may and preferably is all located in the studio.

' Suppose the arrangement is such that with an unmcdulated carrier the length of line or sweep on the fluorescent screen at 8 is a quantity q.. Then whenthe carrier is fully modulated the length of line will be a quantity 2a. Suppose the apparatus` is such that 80% modulation is regarded as the maximum permissib1eviously if it be desired to prevent under-modulation this may be effected by suitably arranging a mask and gain controlling photo-cell unitso that the gain as respects modulation signals is automatically increased if the length of line or fsweep on the fluorescent screen of the tube falls below a predetermined minimum. Ob viously, by a suitable arrangement of cells and mask, it is possible automatically. to prevent both over-modulation and under-modulation.

It will be appreciated that an advantage of the present invention is that there is considerable scope for variation as to adjustment to meet varying conditions likely to be encountered in practice especially if a density wedge or.

wedges are used at lll. The time constants of the various circuits may be made adjustable and the degrees of amplification of the ampliers, e. g., the degree of amplification of the direct current amplifier for the cell, may also be made capable of adjustment.

By suitably disposing the light control device and photo-cell apparatus it is possible so to arrange matters that one end of the sweep on the uorescent screen of the tube is conveniently visible to the eye of an observer, and if this be done the apparatus will also constitute a visual indicator of the degree of modulation.

I claim:

In a modulation control system to be used with signaling'means wherein wave energy is,m odu lated as to amplitude by signal wave energy, a transmitter, a source of modulating potentials, a modulating potential amplifier connecting said source of modulating potentials to said. transmitter, a gain control circuit for said amplifier, amplifying means excited by modulated energy radiated from said transmitter, a cathode ray tube having defiecting plates connected with said amplifying means, said cathode ray tube producing a ray the extent of deflection of which is a function of the amplitude of the'am- 'plied wave energy radiated from the transmitter, a light sensitive device adjacent said cathode ray tube to be controlled by the ray produced thereby, a direct current amplifier connecting said light sensitive device to said gain control circuit of said modulating potential ampliiier, and a light control device in the form of a density Wedge interposed between said light sensitive device and said cathode ray tube or limiting the time during which said ray produced in said cathode ray device reaches said light sensitive device to thereby control the gain of said modulating pcential amplier.

OSWOLD EDWARD KEALL. 

